Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<p>Click the button to test if a variable is undefined.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction() {
    var x;

    if (x === undefined) {
        txt = "x is undefined";
    } else {
        txt = "x is defined";
    }
    document.getElementById("demo").innerHTML = txt;
}
</script>

</body>
</html>